DMX (Digital Multiplex) is a lighting control standard for use in applications such as theatre and concert lighting systems. It offers control of 8-bit values for 512 addresses, updated 44 times per second or faster. DMX defines support for many types of light fixtures, from simple 8-bit dimmed white lamps to RGB moving head lights with strobos and gobos.
Referring to FIGS. 1 to 3, a controller 8 transmits lighting control commands over a lighting network 7 with the commands packaged into DMX frames 26, and with the different frames being transmitted one-after-another at a different respective times in a temporal sequence. The lighting network 7 comprises one or more (slave) lighting devices 10 (shown in FIGS. 1 as 101, 102, and 10N) that are connected to the controller 8 via a suitable bus or interconnect. A DMX network 7 is sometimes referred to as a “universe”. Note also that each lighting device 10 may be allocated one or more of the DMX addresses within a given system, i.e. there need not necessarily be only one address per lighting device. Thus a given device 10 may use a plurality of DMX addresses (e.g. a contiguous range of addresses in the
DMX address space), for instance with a different address being assigned for controlling each of a plurality of different functions of the same device.
As shown in FIG. 2, each DMX frame 26 comprises: a start code 23 to signal the start of the frame, followed by 512 one-byte time slots 25, followed by a break period 27 to separate between adjacent frames 26 in the sequence. Each time slot 25 corresponds to a different DMX address, so that when a DMX frame is issued over the network, the byte of data in that slot is delivered to the relevant address. I.e. the position of a time slot 25 within the frame 26 determines the device and function to be controlled, while the data value of the byte in the slot specifies the control set point for that device or function. Thus a given DMX command may transmit a respective byte to up to 512 different addresses within the space of a single frame (but only one byte per address per frame).
DMX can be used to control a variety of different output functions of a lighting device, for example: to turn a lighting device on or off, to dim the output intensity up or down, to vary the spectrum of the light output, or to change the direction of the lighting device (e.g. to pan and/or tilt the device).
However, DMX is a unidirectional protocol such that, using DMX, the controller 8 can only send data to the lighting devices 10 and cannot not receive any data back from the lighting devices. RDM (Remote Device Management) is a protocol that has been added to the DMX light control protocol in order to enable status feedback (amongst other features). It is used to obtain real-time lamp driver feedback while dynamic scenes are rendered.
As shown in FIG. 3, RDM frames are interleaved with DMX frames by means of time multiplexing. An RDM query cannot be performed over the lighting network 7 at the same time as a DMX frame. Instead, in the space of a DMX frame, the controller 8 transmits an RDM GET or SET command to a lighting device 10 and receives back a corresponding response, all over the same lighting network infrastructure 7 as used for the DMX frames. The RDM traffic is distinguished from the DMX frames by a different start code 23. The RDM SET command allows the controller 8 to configure a lighting device 10 at a certain DMX address and receive an acknowledgement back in response. The acknowledgement from the receiver 10 back to the controller 8 is done within the one DMX frame, so the RDM sent from the controller 8 and the reply are all within the time needed for one single DMX frame. For example an RDM SET command may change the DMX address of the device, change a mode of the device, or invert pan and tilt. The RDM GET command performs a status query directed to a certain RDM address (the UID, a unique ID given at manufacture much like a MAC address), and if a device 10 is present at that address it will return an answer back to the controller 8. The answer from the receiver 10 back to the controller 8 is done within the one DMX frame, so again the RDM sent from the controller 8 and the reply are all within the time needed for one single DMX frame. For example the status query may ask whether a device is present at a certain DMX address, what type of function or device is at a certain address (e.g. a dimmer), what is the current operating temperature of the device, or is there a fault to report.
ArtRDM extends the RDM standard to use over an IP network. RDM SET and GET commands, normally executed over RS-485, are replaced by ArtRDM SET and GET IP packets.
Referring to FIG. 4, in an ArtRDM system the controller 8 is arranged to act as a proxy which receives ArtRDM SET and GET IP packets from an external terminal 2 over an Internet Protocol (IP) network 6. The ArtRDM IP packets are then converted to regular RDM commands at the controller 8, and forwarded onwards in this form over the lighting network 7. The controller 8 also receives back the respective response over the lighting network 7, and then converts this to an IP packet to return to the originating terminal 2 over the IP network 6.
DMX, RDM and ArtRDM can be used in various applications, and are especially useful in real-time applications such as stage lighting where dynamic, real-time changes in the light scene are required. For example, from theatre shows it is well known that dynamic light effects can contribute significantly to the impact of the show. People become exited and experience a more intense atmosphere compared to a situation where only static lighting is used.